Automotive air conditioners use chlorofluorocarbon gases, such as HFC-134a (R-134a), as refrigerants.
From the viewpoint of improving ride comfort, rubber hoses, which are superior in absorbing vibration, are used for automotive air-conditioning piping. In order to prevent the refrigerant from leaking, the rubber hose has a structure including an innermost polyamide resin layer superior in gas barrier property and in vibration durability such as impulse resistance. The hose is also provided with a rubber inner tube layer on the polyamide resin layer, a reinforcing fiber layer made of PET or any other organic fiber on the rubber inner tube layer, and further a weather-resistant EPDM rubber layer on the reinforcing fiber layer (Patent Literature 1).
A refrigerant transporting hose has been proposed whose innermost gas barrier layer is made of a polyamide resin containing a polyolefin elastomer as a flexibility-imparting agent to impart refrigerant permeation resistance and flexibility (Patent Literature 2).
However, polyamide resins can be undesirably degraded by chlorofluorocarbons used as refrigerants, or oil from a compressor. This disadvantage cannot be solved even by adding a polyolefin elastomer. Thus, the known refrigerant transporting hoses still have a remaining issue in terms of durability.
For example, if even a trace amount of acid component is present in an air conditioning system, the polyamide resin composition can be seriously degraded by the acid component under practical conditions of high temperature and high pressure, and may become unusable. Such an acid component can be an extreme pressure agent contained in a compressor oil, enclosed together with a refrigerant. Therefore, some known refrigerant transporting hoses cannot exhibit sufficient durability in practice and may become unusable, depending on environmental conditions and the type of oil used in the air conditioner.
On the other hand, for fiber-reinforced hoses used for automotive oil systems, the hoses including a fiber-reinforced layer between the inner layer made of an acrylic rubber containing an amine-based vulcanizing agent and the outer layer, a technique has been proposed in which hydrotalcite is added to the acrylic rubber to prevent the reinforcing fiber from being degraded by the heat for vulcanization in the manufacturing process (Patent Literature 3). In another technique, for gasohol fuel hoses, hydrotalcite is added to a butyl rubber composition for forming an alcohol-blocking layer around the outer periphery of the inner layer made of a polyamide resin composition to enhance the adhesion between the layers (Patent Literature 4). These techniques, however, do not suggest any solution for the issue the invention should overcome which is to prevent the polyamide resin from being degraded by the refrigerant or compressor oil.
In Patent Literature 5, it is described that by adding magnesium oxide to a polyamide resin outer layer formed on the outer periphery of a fluororesin inner layer, in a predetermined amount relative to the polyolefin elastomer of the outer layer, the adhesion between the polyamide resin outer layer and the fluororesin inner layer is improved. However, this does not imply a combined use of magnesium oxide and hydrotalcite, and the disclosure is not intended to prevent the degradation of polyamide resin.
Fluororesins have excellent chemical durability. However, fluororesins are highly permeable to gases and do not easily adhere to polyamide resins. To overcome these disadvantages, a refrigerant transporting hose has been proposed which includes an inner resin layer having a double-layer structure including an acid-modified fluororesin layer and a polyamide resin layer (Patent Literatures 6 to 9).
In such a refrigerant transporting hose, the acid-modified fluororesin layer can protect the polyamide resin layer and prevent the degradation with chlorofluorocarbons used as the refrigerant or oil from the compressor because of its superior chemical stability and high adhesion to the polyamide resin layer. In addition, the multilayer structure of the acid-modified fluororesin layer and the polyamide resin layer can provide excellent gas barrier properties.
However, the hose including the inner resin layer having the double-layer structure including an acid-modified fluororesin layer and a polyamide resin layer has a high flexural rigidity and, hence, cannot meet the flexibility requirement for routing hoses in a narrow space. Furthermore, although automotive air conditioning hoses are particularly required to exhibit high durability in a fatigue test such as a repeated pressure test (impulse test), as well as to have flexibility, the hose including the inner resin layer having the double-layer structure of an acid-modified fluororesin layer and a polyamide resin layer is also inferior in durability, such as impulse resistance.
Patent Literature 10 (Japanese Examined Patent Application Publication No. 62-13380) has proposed a method for manufacturing a polyamide resin composition containing a flame retardant containing a halogenated organic compound. In this method, the flame retardant is added to a low-melting-point polyamide to prepare a low-melting-point polyamide master batch in advance, and this low-melting-point polyamide masterbatch is mixed with a high-melting-point polyamide. In this method, a flame retardant in the form of a low-melting-point polyamide masterbatch is added to a high-melting-point polyamide in order to prevent the degradation resulting from local overheating that may be caused by a high shear force generated when a flame retardant is directly mixed with a high-melting-point polyamide. Thus, the method is not intended for mixing a specific metal compound to a polyamide resin composition containing a polyamide resin and a polyolefin elastomer.